1. Field of the Invention
This invention relates to a luminous display using luminous elements such as organic EL (electro-luminescence), and its driving method.
2. Description of the Related Art
Recently, attention has been paid to an organic EL display as a self-luminous type display. Development of organic materials has advanced, and its service life has increased. Furthermore, it is thin, and is high in luminescence, and it is low in power consumption including its back light. Hence, its screen is improved in definition and increased in size.
The organic EL is a capacitive element. Therefore, it suffer from a problem that, in a simple matrix drive system popularly employed as a matrix display drive method, the parastic capacitance of the luminous element is charged, and the resultant charge makes the luminescence of the element insufficient.
This problem will be described concretely below:
A drive method shown in FIG. 6 is called xe2x80x9ca simple matrix drive system. Anode lines A1 through A256 and cathode lines B1 through B64 are arranged in matrix. At the intersections of the anode lines and the cathode lines thus connected in matrix, luminous elements E1.1 through E256.64 are connected. The anode lines or the cathode lines are scanned at predetermined time intervals, while, in synchronization with this scan, the other lines are driven with constant currents 21 through 2256 which are employed as drive sources, so that the luminous elements at the desired (optional) intersections are caused to emit light. Each of the constant current sources 21 through 2256 supplies a constant current I.
In the case of FIG. 6, the luminous elements E11 and E12 are turned on. That is, the scanning switch 51 is switched over to 0V (side), and the cathode line B1 is scanned.
For the remaining cathode lines B2 through B64, the scanning switch 52 through 564 function, to apply reverse bias voltage Vcc (10V) to them B2 through B64.
The application of the reverse bias voltage is to prevent current supplied from the constant current sources 21 through 2256 from being applied to the cathode lines which are not scanned, It is preferable that the value Vcc is substantially equal to the voltage value applied between the luminous elements to cause the luminous elements to emit light at a desired instantaneous brightness; that is, a voltage of the luminous element which are connected between a constant current source and ground.
The anode lines A1 and A2 are connected through drive switches 61 and 62 to the constant current sources 21 and 22, and shunt switches 71 and 71 are kept opened. For the remaining anode lines A3 through A256, the constant current sources are opened, and the shunt switches 73 through 7256 are at ground potential.
Accordingly, in the case of FIG. 6, the luminous elements E1.1 and E2.1 are biased forwardly, so that drive currents from the constant current sources flow as indicated by the arrows in FIG. 6, whereby only two luminous elements E1.1 and E2.1 emit light.
The operations of the scanning switches 51 through 564, the drive switches 61 through 6256, the shunt switch 71 through 7256 are controlled by a luminescence control circuit 4+ to which luminous data are applied.
With the aid of the scanning switches 52 through 564, reverse bias voltage is applied to first terminals of the luminous elements connected at the intersections of the cathode lines B2 through B64 and the anode lines A1 and A2, while the constant current sources 21 and 22 supply a voltage, which is substantially equal to the reverse bias voltage, to the second (remaining) terminals thereof. Therefore, no current flows in the luminous elements. Accordingly, no parastic capacitances of the luminous elements are charged.
Reverse bias voltage is applied to the luminous elements connected at the intersections of the cathode lines B through B64 and the anode lines A3 through A256. Therefore, the parastic capacitances (the capacitors shaded) of the luminous elements are reversely charged as indicated in FIG. 6 (the potential on the side of cathodes of the element being higher).
When, under the condition that the parastic capacitances are reversely charged, the cathode lines are scanned to cause the next luminous element to emit light, then the period of time required for the next luminous element to activate, and accordingly, it is impossible to perform a high speed scanning operation. This will be described with reference to FIGS. 7A and 7B.
FIGS. 7A and 7B show only the luminous elements E3,1 through E3,64 connected to the anode line A3 in FIG. 6. FIG. 7A is for a description of the scanning of the cathode line B1, and FIG. 7B is for a description of the scanning of the cathode line B2. In this connection, let us consider the case where, when the cathode line B1 is scanned, the light emission of the luminous element E3,1 is not carried out, and when the cathode line B2 is canned, the light emission of the luminous element E3,2 is carried out.
As shown in FIG. 7A, in the case where, when the cathode line B1 is scanned, the anode line A3 is not driven, the luminous elements E3,2 through E3,64 (other than the luminous element E3,1) connected to the cathode line B1 which is being scanned are charged as shown in FIG. 7A by the reverse bias voltage Vcc applied to the cathode lines B2 through B64.
As shown in FIG. 7B, if, when the scanning is shifted to the cathode line B2, the anode line A3 is driven to cause the luminous element E3,2 to emit light, then not only the parastic capacitance of the luminous element E3,2 is charged, but also current flows to the parastic capacitances of the luminous elements E3,3 through E3,64 connected to the other cathode lines B3 through B64 as indicated by the arrows; that is, those parastic capacitances are charged.
On the other hand, a luminous element has a characteristic that its luminescent brightness changes with a voltage across it. Hence, if the voltage across it is not increased to a predetermined value, the steady light emission (the light emission with a desired instantaneous brightness) thereof is not achieved.
In the case of the conventional drive method, as shown in FIGS. 7A and 7B, when the anode line A3 is driven to cause the luminous element E3,2 to emit light which is connected to the cathode line B2, then not only the parastic capacitance of the luminous element E3,2 to be caused to emit light but also the other luminous elements E3,3 through E3,64 connected to the anode line A3 are charged. Therefore, it takes time to charge the parastic capacitance of the luminous element E3,2 to be caused to emit light; that is, it is impossible to quickly increase the voltage across the luminous element E3,2 to a predetermined value which is connected to the cathode line B2.
Accordingly, the conventional method is disadvantageous in that the time required for a luminous element to emit light is slow, and it is impossible to perform a high speed scanning operation.
In order to solve this problem, the present Applicant has proposed the following drive method under Japanese Patent Application No. 38393/1996: As shown in FIG. 8, during the period of time between the accomplishment of a scanning operation and the shifting the scanning operation to the next cathode line, all the drive switch 61 through 6256 are turned off, all the scanning switches 51 through 564 and all the shunt switches 71 through 7256 are switched over to 0V side, so that the resetting operation with 0V is effected, whereby the parastic capacitances of the luminous elements are discharged. The proposed method functions as described above.
In the above-described conventional drive method, the parastic capacitances of the luminous elements E3,2 through E3,64 charged by the reverse bias voltage Vcc during the scanning of the cathode line B1 is discharged before the scanning is shifted to the cathode line B2. Therefore, at the moment the scanning is shifted to the cathode line B2, the circuit is as shown in FIG. 9. In this case, the parastic capacitances of all the luminous elements have been discharged. Therefore, currents from a plurality of routes shown in FIG. 9 flow to the luminous element E3,2 to be caused to emit light next, so that the luminous element E3,2 is quickly caused to emit light.
FIGS. 10 and 11 show another conventional drive method, which is different from the above-described one in a luminous element resetting operation.
In the drive method, drive switches 61 through 6256 are 3-contact type change-over switches. The first contacts are connected to nothing (open), the second contacts are connected to constant current sources 21 through 2256, and the third contacts are connected to the power source Vcc=10V.
In the case where the luminous elements E1,1 and E2,1, the state of the circuit is as shown in FIG. 10, being the same as that shown in FIG. 6.
The two luminous elements E1,1 and E2,1 are caused to emit light. And before, in order to cause the next luminous element to emit light, the cathode line B2 is scanned, as shown in FIG. 11 all the shunt switches 71 through 7256 are turned off, and all the scanning switches 51 through 564 are switched over to the reverse bias voltage side, and all the drive switches 61 through 6256 are switched over to the third contact side.
As a result, all the anode lines A1 through A256, and all the cathode lines B1 through B64 are shunt with the constant voltage source, so that the parastic capacitances of all the luminous elements are instantaneously discharged.
That is, in the above-described second conventional method, during the period of time between the accomplishment of the scanning of a (an optional) cathode line and the shifting of the scanning operation to the next cathode line, all the luminous elements are reset so as to discharge the parastic capacitances of the luminous elements. The time between the supplying of drive current to a luminous element to be caused to emit light next and the emission of light thereof is reduced; that is, a high speed scanning operation is carried out.
As the display panel is increased in size and in definition, the number of luminous elements is increased, and the cathode lines and the anode lines connecting those luminous elements is elongated and thinned. Since the cathode line is made of a metal line, usually it has a low resistance. In the case where the cathode line and the anode line are elongated and thinned, then they are increased in resistance as much.
The above-described drive method pays no attention to the resistance of the cathode lines; however, if the resistance is increased, the following problem which cannot be disregarded is involved.
This problem will be described with reference to FIG. 12. FIG. 12 shows a part of FIG. 6.
In FIG. 12, the resistance r1 of the cathode lines B1 through B64 between the scanning switches 51 through 564 and the luminous elements E1,1 through E1,64 can be regarded as about zero (0). The resistance of the cathode lines are gradually increased in proportion to the distances from the scanning switches 51 through 564. And its resistance r 256 becomes maximum between the scanning switches 51 through 564 and the luminous elements E256,1 through E256,64.
Let us consider the case where the parastic capacitances of the luminous elements are discharged by the above-described resetting operation, the scanning is shifted from the cathode line B1 to the cathode line B2, and in order to cause the luminous elements E1,2 and E2,256 to emit light the anode lines A1 through A256 are connected to the constant current sources 21 through 2256.
When the scanning is shifted, current flows in the luminous element E1,2 from the side of the luminous elements E1,1, and E1,3 through E1,64; however, since the resistance of the cathode line B2 between luminous element E1,2 and the scanning switch 52 is about zero (0), there is no voltage drop due to the resistance of the cathode line B2. Therefore, the voltage applied across the luminous element E1,2 becomes approximately Vcc immediately, and the latter E1,2 is charged in correspondence to Vcc. Hence, the voltage across the luminous element E1,2 can be increased to the desired value Vcc, and immediately the light emission is carried out with a desired instantaneous luminance.
On the other hand, when current flows into the luminous element E256,2 from the side of the luminous elements E256,1 and E256,3 through E256,64 after the scanning is switched, axe2x80x94voltage drop V256 is caused by the resistance r256 of the cathode line B2.
Therefore, the voltage across the luminous element E256,2 is Vccxe2x88x92V256, and the parastic capacitance of the latter E256,2 is charged as much. Accordingly, immediately after the switching of the scanning the voltage across the luminous element E256,2 is not the predetermined value yet, and therefore the light emission is not carried out with a desired instantaneous luminance. In order to perform the light emission at the desired instantaneous luminance, the current supplied from the constant current source 2256 must be applied thereto until the voltage across the luminous element reaches the predetermined value Vcc. For this purpose, all the luminous elements E256,1 through E256,64 must be charged until the potential of the anode line A256 reaches Vcc+V256; however, this operation will take a lot of time.
As is apparent from the above description, the luminous element E256,2 cannot obtain a sufficiently high luminance during its selected period, and is deviated in luminance from the luminous element E1,2. Those factors makes the screen unclear.
As was described above, because of the resistance of the cathode lines, the element located far from the scanning switches 51 through 564 is insufficient in luminance when compared with the element located near those scanning switches. That is, the display panel is not uniform in the distribution of luminance.
In view of the foregoing, an object of the invention is to provide a luminous display which realizes a display panel in which the elements are uniform in luminance, and to provide a drive method thereof.
According to a first aspect of the invention, there is provided a method of driving a luminous display in a simple matrix drive system in which luminous elements are connected at the intersections of a plurality of anode lines and a plurality of cathode lines which are arranged in matrix form, the cathode lines or the anode lines are employed as scanning lines, while the others are employed as drive lines, and while the scanning lines are scanned with a predetermined period, in synchronization with the scanning operation drive sources are connected to desired drive lines to cause luminous elements to emit lights which are connected at the intersections of the scanning lines and the drive lines, said method comprising the step of:
scanning one scanning line; and
applying an offset voltage to the luminous elements to charge the luminous elements during a period of time when the scanning of the one scanning line is switched over to the scanning of a succeeding scanning line after the scanning of the one scanning line has been completed.
According to a second aspect of the invention, there is provided a method of driving a luminous display according to the first aspect, wherein said offset voltage applying step comprises the steps of:
grounding the scanning lines; and
connecting the drive lines to voltage sources different from the drive sources.
According to a third aspect of the invention, there is provided a method of driving a luminous display according to the first aspect, wherein the offset voltages are set to values corresponding to drop voltages across resistances between the luminous elements of the scanning lines and the ends of the scanning lines.
According to a fourth aspect of the invention, there is provided a method of driving a luminous display according to the first aspect; wherein the offset voltages are set to values corresponding to resistances between the luminous elements and the ends of the scanning lines.
According to a fifth aspect of the invention, there is provided a method of driving a luminous display according to the first aspect, wherein said offset voltage applying step comprises the step of applying bias voltages to the scanning lines which are not scanned are applied, of the plurality of scanning lines; and
grounding the drive lines which are not driven, of the plurality of drive lines.
According to a sixth aspect of the invention, there is provided a method of a luminous display according to the first aspect, wherein the luminous elements are organic EL elements having parastic capacitances.
According to a seventh aspect of the invention, there is provided a luminous display in a simple matrix drive system, said display comprising:
a plurality of anode lines;
a plurality of cathode lines, said cathode lines and said anode lines being arranged in matrix form, ones of said cathode lines and said anode lines being employed as scanning lines, and the others being employed as drive lines;
a plurality of luminous elements connected at intersections of said anode lines and said cathode lines;
bias voltage applying means for applying bias voltage to the scanning lines, each of the scanning lines is connected to one of said voltage applying means and the ground;
constant current sources for supplying drive currents to said luminous elements; and
voltage sources for applying offset voltages to said luminous elements, said anode lines being connected to one of said constant current sources, said voltage sources and the ground;
wherein said scanning lines are scanned with a predetermined period, desired drive lines are driven in synchronization with the scanning operation to cause said luminous elements to emit lights.
According to an eighth aspect of the invention, there is provided a luminous display according to the seventh aspect, wherein during a period of time before, after the scanning of an optional scanning line is accomplished, the scanning is switched over to the scanning of the next scanning line, the plurality of drive lines are connected to the voltage sources, while the scanning lines are grounded, so that the luminous elements are charged.
According to a ninth aspect of the invention, there is provided a luminous display according to the seventh aspect; wherein the offset voltages are set to values corresponding to drop voltages across resistances between the luminous elements of the scanning lines and the ends of the scanning lines.
According to a tenth aspect of the invention, there is provided a luminous display according to the ninth aspect; wherein the voltage sources are variable voltage sources, and comprises:
voltage determining means which, according to the light emission states of all the luminous elements connected to the cathode line which is to be scanned next, determine offset voltages which are to be applied to those luminous elements; and
voltage control means for controlling supply voltage values of the variable voltage sources so as to apply offset voltages which are determined by the offset voltage determining means.
According to an eleventh aspect of the invention, there is provided a luminous display according to the seventh aspect, wherein the offset voltages are set in correspondence to resistances between the luminous elements and the ends of the scanning lines.
According to a twelfth aspect of the invention, there is provided a luminous display according to the seventh aspect, wherein during a scanning period of the scanning lines, the lines which are not scanned are connected to the bias voltage applying means, and the lines which are not driven are grounded.
According to a thirteenth aspect of the invention, there is provided a luminous display according to the seventh aspect, wherein the luminous elements are organic EL elements having capacitances.
In the luminous display driving method according to a simple matrix drive system in which luminous elements are connected at the intersections of a plurality of anode lines and a plurality of cathode lines which are arranged in matrix form, the cathode lines or the anode lines are employed as scanning lines, while the others are employed as drive lines, and while the scanning lines are scanned with a predetermined period, in synchronization with the scanning operation drive sources are connected to desired drive lines thereby to cause luminous elements to emit lights which are connected at the intersections of the scanning lines and the drive lines; according to the invention, during a period of time before, after the scanning of an optional scanning line is accomplished, the scanning is switched over to the scanning of the next scanning line, an offset voltage is applied to charge the luminous elements. Hence, the fluctuation in the light emission start time of the luminous elements which is due to the resistances of the cathode lines is minimized, and the luminous display is displayed so that the person is able to observe the display with ease.
Furthermore, in the luminous display according to a simple matrix drive system in which luminous elements are connected at the intersections of a plurality of anode lines and a plurality of cathode lines which are arranged in matrix form, the cathode lines or the anode lines are employed as scanning lines, while the others are employed as drive lines, and while the scanning lines are scanned with a predetermined period, in synchronization with the scanning operation drive sources are connected to desired drive lines thereby to cause luminous elements to emit lights which are connected at the intersections of the scanning lines and the drive lines; according to the invention, each of the scanning lines is connectable to bias voltage applying means adapted to apply bias voltage or ground, and the anode lines are connectable to one selected from a group consisting of constant current sources adapted to supply drive currents to the luminous elements, voltage sources adapted to apply offset voltages to the luminous elements, and ground. Hence, the fluctuation in the light emission start time of the luminous elements which is due to the resistances of the cathode lines is minimized, and the luminous display is displayed so that the operator is able to observe the display with ease.